1. FIELD OF THE INVENTION
This invention relates to a method of producing GaAs single crystal doped with boron (B) and more particularly, it is concerned with a method of growing GaAs crystal doped with boron from GaAs melt covered with B.sub.2 O.sub.3 melt as a liquid encapsulant, thus lowering the dislocation density (etch pit density) in the grown crystal.
2. DESCRIPTION OF THE PRIOR ART
The important technical problem of GaAs single crystals (whose use has lately been developed as basic material for semiconductor lasers, GaAs IC or optoelectronic GaAs IC) is that it is difficult to obtain a large-sized dislocation-free crystal such as silicon (Si).
GaAs has a smaller mechanical strength at a high temperature than Si. Thus, it is considerably difficult to produce a single crystal with a cross section of 5 cm.sup.2 or more and a dislocation density (EPD).ltoreq.2,000 cm.sup.-2 on a commercial scale (see Japanese Patent application OPI No. 18471/1976 and 18472/1976). A large-sized single crystal of the so-called dislocation-free (DF) class with EPD.ltoreq.100 cm.sup.-2 can be obtained in only GaAs crystal doped with Si in a quantity of 1.5.times.10.sup.18 to 5.5.times.10.sup.18 atoms per cm.sup.3 using the three-temperature-zone horizontal Bridgman (HB) method (Japanese Patent application OPI No. 62200/1977).
Since the range of its use as GaAs IC is increasing of late, a large-sized, circular low-dislocation GaAs single crystal has eagerly been demanded and it is thus required to realize a low dislocation by a technique such as a liquid encapsulated Czochralski method (LEC method), not by the prior art boat growth method (HB method, etc.).
As such a technique for realizing a low dislocation of GaAs by LEC method, in particular, the so-called "impurity hardening method" has been thought promising and is disclosed in Japanese Patent Application OPI No. 63065/1977. This describes that a low dislocation of an object single crystal can be realized by incorporating in the crystal one or more impurities selected in such a manner that the bond energy (single bond energy) between the added impurity atoms and the constitutional atoms of the object single crystal is larger than the bond energy of the object crystal in a total concentration of 1.times.10.sup.-3 atom % or more. In the case of GaAs, 1.times.10.sup.-3 atom % corresponds to about 4.4.times.10.sup.17 atoms per cm.sup.3. It is further described therein that the above described method is available for not only the LEC method but also other crystal growth methods such as three-temperature-zone method and HB method. Furthermore, phosphorus (P), aluminum (Al), oxygen (O) nitrogen (N), boron (B), sulfur (S) and zinc (Zn) are proposed as examples of the impurity to be added to GaAs.